Surface deformation caused by sub-surface and internal defects can often be the cause of failure in structural components. Small and reliable sensors for remote detection of angular and/or spatial displacements of a material are important for many practical applications. For example, accurate detection of cracks in aircraft is important.
Various nondestructive evaluation (NDE) techniques have been developed for detecting and evaluating defects including surface discontinuities, surface flaws, voids, and cracks on the surface or in the body of materials. Most NDE techniques have been developed for investigating metallic materials because most load bearing parts in industry are made of metallic materials. Metallic materials can be categorized in magnetic and nonmagnetic materials depending on their magnetic permeability. For magnetic metallic materials such as steel, techniques such as eddy current, magnetic flux leakage, magnetic Barkhausen noise, and magnetic particle inspection can be employed. For nonmagnetic materials, dye penetrant, radiography, and ultrasonic techniques can be used.
The eddy current technique can be used for both magnetic and nonmagnetic materials, but the materials under investigation must have sufficient electrical conductivity to allow electromagnetically induced currents to be generated in order for the eddy current technique to be viable. The other techniques described above do not work on both magnetic and non magnetic metallic materials.
Compared with the development of magnetic and electromagnetic techniques, the application of optical methods for NDE has been less widespread. Optical methods have the advantage that they can provide inspection without needing a direct contact with the test material. Any changes on the surface can be monitored by a reflected light beam signal. This signal can be detected by a photodiode or a CCD. In addition to the capability for remote measurement of defects, optical methods provide high stability with respect to electromagnetic interference, high spatial resolution and the capability of performing over a wide frequency range.
The existing magneto-optic technique for NDE applications employs magneto-optic film that can sense stray magnetic fields from a surface defect. The technique is performed by placing a magneto-optic film in close proximity to the surface of the material. Magnetic leakage fields from a surface defect cause rearrangement of domain structure in the magneto-optic film, giving rise to contract in the domain images of the magneto-optic film which are indicative of defects in the surface. The problem with this technique is that it is insufficiently sensitive for surface deformation caused by sub-surface and internal defects (i.e., micro-scale surface deformation) due to negligible magnetic flux leakage fields. Furthermore, this technique is not applicable to non-conducting materials (e.g., ceramics, polymer-matrix composites, etc.) due to the absence of magnetic flux leakage fields emanating from the non-conducting materials.